Bimodality in low luminosity E and S0 galaxies

Abstract

Stellar population characteristics are presented for a sample of low luminosity early-type galaxies (LLEs) in order to compare them with their more luminous counterparts. Long-slit spectra of a sample of 10 LLEs were taken with the ESO New Technology Telescope, selected for their low luminosities. Line strengths were measured on the Lick standard system. Lick indices for these LLEs were correlated with velocity dispersion (sigma), alongside published data for a variety of Hubble types. The LLEs were found to fall below an extrapolation of the correlation for luminous ellipticals and were consistent with the locations of spiral bulges in plots of line strengths versus sigma. Luminosity weighted average ages, metallicities and abundance ratios were estimated from chi-squared fitting of 19 Lick indices to predictions from simple stellar population models. The LLEs appear younger than luminous ellipticals and of comparable ages to spiral bulges. These LLEs show a bimodal metallicity distribution, consisting of a low metallicity group (possibly misclassified dwarf spheroidal galaxies) and a high metallicity group (similar to spiral bulges). Finally, they have low alpha-element to iron-peak abundance ratios indicative of slow, extended star formation.

26 pages, 9 figures and 9 tables. PDF file; accepted for publication in MNRAS

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